Sealable, transparent polyolefinic multilayer film

Abstract

A sealable, transparent polyolefinic multilayer film composed of a polypropylene base layer, a non-sealable surface layer, and a sealable surface layer, and process therefor. The sealable surface layer has a low minimum sealing temperature and is made from a copolymer of propylene and ethylene or butene-1 units and/or a terpolymer of ethylene, propylene and butene-1 units. The non-sealable layer is a combination of a propylene homopolymer, a slip agent, and an anti-blocking agent. The process includes coextruding the three layers, biaxially stretching the coextruded film, and subjecting the non-sealable layer to a corona-discharge treatment.

What is claimed is: 
     
       1. A sealable, transparent polyolefinic multilayer film, comprising: a polypropylene base layer;   a sealable surface layer disposed on one surface of said base layer and comprising an ethylene-propylene-butene-1 terpolymer and a propylene-butene-1 copolymer, corresponding to a content of from about 0.1% to about 7% by weight of ethylene, from about 53% to about 89.9% by weight of propylene and from about 10 to about 40% by weight of butene-1, based on the total weight of the olefin resin composition; and   a non-sealable surface layer disposed on the other surface of said base layer and comprising from about 96.5% to about 99.6% by weight of a propylene homopolymer, from about 0.3% to about 2.5% by weight of a slip agent, and from about 0.1% to about 1.0% by weight of an anti-blocking agent.   
     
     
       2. A packaging film produced from the multilayer film of claim 1, wherein said surface layers have a thickness of less than about 3 μm. 
     
     
       3. A packaging film as claimed in claim 2, wherein said multilayer film is biaxially stretched, and wherein the biaxially structured multilayer film has a total thickness of from about 10 to about 50 μm. 
     
     
       4. A multilayer film as claimed in claim 1, wherein said sealable surface layer further contains at least one additive selected from the group consisting of a low molecular weight resin, said resin being compatible with the copolymer or terpolymer of said sealable surface layer, and a propylene homopolymer, each of said at least one additive present in an amount of from about 5% to about 15% by weight of said sealable surface layer. 
     
     
       5. A multilayer film as claimed in claim 1, wherein said non-sealable surface layer is corona-treated to improve its printability. 
     
     
       6. A multilayer film as claimed in claim 1, wherein said film is oriented by a biaxial stretching process. 
     
     
       7. A multilayer film as claimed in claim 1, wherein said slip agent is a polydiorganosiloxane. 
     
     
       8. A multilayer film as claimed in claim 7, wherein said polydiorganosiloxane is selected from the group consisting of polydialkylsiloxane and polyalkylphenylsiloxane. 
     
     
       9. A multilayer film as claimed in claim 8, wherein said polydiorganosiloxane is selected from the group consisting of a polydimethylsiloxane and a polymethylphenylsiloxane. 
     
     
       10. A multilayer film as claimed in claim 9, wherein said polydiorganosiloxane has a kinematic viscosity of from about 10 3  to about 10 6  mm 2  /s at 25° C. 
     
     
       11. A multilayer film as claimed in claim 9, wherein said polydiorganosiloxane has a kinematic viscosity of from about 20,000 to about 30,000 mm 2  /s at 25° C. 
     
     
       12. A multilayer film as claimed in claim 1, wherein said anti-blocking agent is an inorganic pigment. 
     
     
       13. A multilayer film as claimed in claim 12, wherein said inorganic pigment is a silicate. 
     
     
       14. A multilayer film as claimed in claim 12, wherein the inorganic pigment contained in said non-sealable surface layer comprises a platelet-shaped silicate having an average particle diameter of from about 0.1 to about 3 μm. 
     
     
       15. A multilayer film as claimed in claim 14, wherein the inorganic pigment has an average particle diameter of from about 0.2 to about 1.5 μm. 
     
     
       16. A process for preparing a sealable, transparent polyolefinic multilayer film, comprising the steps of: coextruding a base layer and a separate surface layer on each side of said base layer to form the multilayer film, one of said surface layers being sealable and the other of said surface layers being non-sealable;   biaxially stretching said multilayer film in longitudinal and transverse directions; and   subjecting said non-sealable surface layer to a corona-discharge treatment;   wherein said base layer is a polypropylene, said sealable surface layer comprises an ethylene-propylene-butene-1 terpolymer and a propylene-butene-1 copolymer, corresponding to a content of from about 0.1% to about 7% by weight of ethylene, from about 53% to about 89.9% by weight of propylene and from about 10 to about 40% by weight of butene-1, based on the total weight of the olefin resin composition; and said non-sealable surface layer comprise from about 96.5% to about 99.6% by weight of a propylene homopolymer, from about 0.3% to about 2.5% by weight of a slip agent, and from about 0.1% to about 1.0% by weight of an anti-blocking agent.   
     
     
       17. A process as claimed in claim 16, wherein said corona-discharge treatment is performed at an intensity sufficient to obtain a surface tension equivalent to between about 36 and about 40 mN/m in a comparable surface layer lacking said slip agent. 
     
     
       18. A process as claimed in claim 16, wherein said biaxially stretched film has a thickness of from about 10 μm to about 50 μm.